APEX

I need data in one table column are filled automatically with APP_USER when user uploads data with loader wizard (I do not want user to populate data for this column). So I create Transformation rule. I choose column name, then I create 'PLSQL Function body' as Rule type. Function body is:

DECLARE
 somevariable varchar2(100);
BEGIN
somevariable := V('APP_USER');
 return somevariable;
END;

But is does nothing. All rows are still empty in this column. Why?
And one more question; how to make this solumn hidden (user will not see this column in 'target column' combo-box)? Regards